The shy and secretive Hawaiian duck is one of several species belonging to the North American Mallard complex, a group of closely related ducks (4). Although similar in appearance to the much more common and widespread mallard duck, the genetically distinct Hawaiian duck is smaller, behaves differently, and has different plumage (2) (4) (5) (6). Male and female Hawaiian ducks are mottled brown overall, but while the larger male has a predominately dark, olive bill and bright, orange feet, the female has a dark bill with orange or fleshy tones, and light orange feet (2) (4) (6) (7). The familiar quack of the mallard duck is softer and heard less often in the Hawaiian species (2) (4). Hybrids between the two species share characteristics of both parents (2) (6).

Once common on most of the main Hawaiian Islands, the vast majority of Hawaiian ducks now only occur on Kauai, Niihau, and the upper elevations of Hawaii, with the largest numbers at the Hanalei National Wildlife Refuge on Kauai. Captive reared Hawaiian ducks were reintroduced to Hawaii, Oahu, and Maui, but on all three islands there is now a high proportion of hybrids (2) (4) (6).

Wary by nature, especially when nesting or molting, the Hawaiian duck often occurs alone or in pairs (7). It is a strong, agile flier and is believed to make daily and seasonal altitudinal movements. Although inter-island movements are unknown, patterns in abundance suggest this species does move between the islands in response to rainfall and food availability (4) (8). The Hawaiian duck is an opportunistic feeder, foraging mainly in shallow water, where it takes aquatic insects, molluscs, crustaceans, seeds and other plant matter (2) (4) (6).

The reproductive ecology of this duck is poorly known (6), but breeding has been documented year-round with a peak between December and May on Kauai, and March and June on Hawaii (2) (3). The female normally builds a nest away from human disturbance, in herbaceous upland vegetation near a wetland or stream, and lays up to ten eggs (4) (6). Following an incubation period of around 30 days, the precocial chicks hatch and trail their mother to the safety of water (3).

By the mid 20th century, the combined impacts of predation by non-native animals, habitat loss for agriculture and urban development, and over-hunting, decimated the once abundant Hawaiian duck to around 530 individuals (2). Fortunately, with protection and prohibition of hunting, the population recovered to around 2,000 at the beginning of this century (2) (5) (6). However, today, a much more subtle dynamic presents the greatest threat to the species survival. Non-native mallards were first introduced to Hawaii for ornamental ponds and farming in the 1800s, and in the 1950s and 1960s hundreds were imported to stock hunting areas. Since then, cross-breeding between the two closely related species has had a dramatic influence on the number of pure Hawaiian ducks, particularly on Oahu and Maui. Furthermore, it is now suspected that even the core populations on Kauai and Niihau contain hybrids (2) (5). In addition to hybridisation, several other more conspicuous threats continue to have an impact on this species including predation by introduced mammals, habitat loss, wetland modification by alien plants, and human disturbance (2).

The establishment of several State and Federal wildlife sanctuaries, in conjunction with legal protection under endangered species laws and a program of re-introductions of captive bred birds, was instrumental in the recovery of the Hawaiian duck population during the latter half of the 20th century (2) (4) (6). In the late 1980s, a ban was placed on the importation of mallards, except for research and exhibition, and was followed in 2002 by the implementation of restrictive import regulations on all birds shipped to the islands, to protect the public from West Nile Virus (2) (5). Even so, the mallard continues to reproduce on the islands and is still sold commercially (5). The priority therefore is to control populations of mallards and hybrids, to prevent irreversible genetic swamping of the Hawaiian duck populations (2). This is to be achieved via a three-pronged approach involving public outreach, humane removal of feral mallards and hybrids, and stronger regulatory controls on non-native, potentially harmful ducks (4). Already underway is an outreach and communications plan and field key for differentiating pure birds from hybrids (4) (5).

Crustaceans: diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.

Hybridisation: cross-breeding between two different species or subspecies.

Hybrids: the offspring produced by parents of two different species or subspecies.

Incubation: the act of incubating eggs, that is, keeping them warm so that development is possible.

Precocial: young that are well developed and mobile at birth or hatching and therefore require less parental care.